import requests
from lxml import etree
url='https://www.52pojie.cn/forum-5-1.html'
html=requests.get(url).text
nr=etree.HTML(html)
a=nr.xpath('//tbody[contains(@id,'normalthread')]/tr/th/a[2]')
print(a[0].attrib['href'])
#//*[@id="normalthread_931785"]/tr/th/a[2]